The bone conduction speaker unit converts a voice signal into a vibration to transmit it to the skull, and the like, being compact and lightweight and capable of being used even under noise, whereby, in recent years, it has been widely used with communication apparatuses, such as a mobile phone, and the like.
In order to transmit a vibration of a diaphragm constituting the bone conduction speaker to the skull, a contact which is abutted against a side face of the head is fixed to the diaphragm, and for efficient transmission of the vibration of the diaphragm to the contact, the contact is fixed to the diaphragm (the plate yoke) in plane contact over a wide area (WO2005/069586A1, and the like). However, in that case, at the time of non-calling, in other words, also when the contact is not pressed against a side face of the head, the contact is vibrated together with the bone conduction speaker being operated, thereby a voice vibration (an air conduction sound) being generated, and thus generation of a certain degree of sound leakage cannot be avoided, presenting a problem.
Then, as a configuration of a bone conduction speaker unit which, at the time of non-calling, can prevent generation of sound leakage and, at the time of calling, can exert the performance comparable to that of a conventional bone conduction speaker, there has been proposed a configuration which includes an accommodating section made of an elastic material, enclosing and supporting the whole of the bone conduction speaker therein, and a movable top face covering the top of the accommodating section and being disposed in contact with a diaphragm of the bone conduction speaker so as to be vibrated therewith, the movable top face having a projection formed on the inner face thereof, and only the projection being in contact with a plate yoke fixed to the diaphragm when the communication apparatus is not in use (Japanese Patent Publication No. 4369976).
For the bone conduction speaker related to this proposal, it has been confirmed that sound leakage is fairly reduced, but to a degree which cannot be the to be sufficient. In other words, the degree of reduction is not so high as to cause the leaked sound to be practically not heard when the ear is brought to near the bone conduction speaker, but to be loudly heard once the ear is pressed against it, thereby a distinct difference in loudness being perceived. In addition, matters, such as forming a projection on the inner face of the movable top face and requiring the degree of contact with the plate yoke to be adjusted, present a problem of an increase in manufacturing cost.